A projection lens wherein light is telecentric on the reducing side of the projection lens is desirable for use in a projection television having a liquid crystal display panel. This is because most liquid crystal displays must be illuminated by light that is normal to the back surface of the liquid crystal display for proper operation of the liquid crystal display, and the use of a telecentric lens prevents unnecessary decrease in the image brightness near the periphery of the image. Even where a telecentric optical system is not required, a long back focus is usually required for there to be room to insert the components of a color composite optical system. As a lens to satisfy such a requirement, the one described in, for example, Japanese Laid-open Patent Application H10-62684, is known. This lens provides a telecentric optical system on its reducing side in which distortion is sufficiently corrected. Thus, there is room for inserting the components of a color composite optical system.
However, as the resolution and luminance of images displayed by liquid crystal projectors have increased, this has generated a need for higher resolution and brighter projection lenses. In making a projecting lens have higher resolution and brightness, the lens inevitably becomes larger. If a lens is focused by shifting the entire lens, as described in the above-noted Japanese Laid-open Patent Application H10-62684, the shifting mechanism itself must be larger as the lens become larger. With a larger shifting mechanism, the necessary driving force is increased and speed is decreased.
The adaptation of such a lens to that of the inner focus type, wherein only certain lens groups of the projection lens rather than the projection lens as a whole are shifted, and wherein focusing is made simple is desired so as to solve this problem An inner focus type of lens is well-known in photographic lenses and video camera lenses, but problems have been encountered regarding telecentric characteristics and the length of back focus when adapting a projection lens to be of the inner focus type.